Plasmas are useful in a variety of device fabrication processes, including etching, deposition, surface cleaning, and ion implantation. Plasmas may be produced by exciting free electrons in a gas to energy levels that are at or above the ionization energy of the gas molecules. The free electrons may be excited by time-varying electric fields, time-varying magnetic fields, or both. In one approach, described in U.S. Pat. No. 5,435,881, a plasma is produced using a two-by-two or larger array of alternating magnetic poles set proximate to an insulating window in a chamber containing a process gas. The magnetic poles are ferromagnetic core coils driven by a radio-frequency power source. The magnetic poles are uniformly spaced from each other and are configured so that adjacent poles have the same magnitude but opposite magnetic polarity. In operation, the varying magnetic fields between adjacent, opposite polarity poles excite free electrons in the chamber to produce a plasma in the regions of the chamber located between adjacent poles.